Infectious Skeletal Disorders in Poultry

Coagulase-positive staphylococci (also see Staphylococcosis) are frequently responsible for bacterial infections in the bones and joints of broiler chickens. Mycoplasma synoviae (see Mycoplasma iowaeInfection in Poultry) may also play a role in infectious bone disorders.

In broilers, bacterial infections are most common in the proximal femur and proximal tibiotarsus in birds >22 days old. In the proximal femur, the condition is also referred to as femoral head necrosis, which is reported to be the most common cause of lameness in broiler chickens. The etiology appears to be vertically transmitted staphylococci, often in combination with infection by immunosuppressive viruses (eg, see Infectious Bursal Disease). Floor eggs have been shown to be common carriers of staphylococci, so their use should be avoided. A high standard of hatchery hygiene can reduce this risk. Formaldehyde fumigation within the hatchers is also likely to help. In addition, hatchery fluff samples (ie, hatching debris such as down feather and egg shell remains) can be examined to monitor for contamination with staphylococci.

Staphylococcal infections in joints and tendons are also seen in breeders. Outbreaks are often due to bacterial infection subsequent to an existing tendinitis. A history of other diseases such as coccidiosis is often associated with an increase in staphylococcal infections in breeders. In some instances, reoviruses may also be isolated. (Also see Viral Arthritis). The virus is vertically transmitted. Vaccines against the condition have been developed.

Escherichia coli is often responsible for flock outbreaks of arthritis and osteomyelitis in broiler chickens and turkeys. Osteomyelitis and arthritis are sequelae of septicemia, and these outbreaks may be associated with enteric or respiratory disease. Infection of bones, joints, and periarticular tissues with E coli, Staphylococcus aureus, or S hyicus is referred to as turkey osteomyelitis complex (TOC) or green-liver osteomyelitis complex. Green discoloration of the liver is often associated with TOC and is mostly observed at the processing plant but is rare in birds that died or are euthanized in the field.

Enterococcus cecorum is a commensal enteric bacterium that can cause epidemics of osteomyelitis, arthritis, and spondylitis in broilers and broiler breeders. Enterococcal spondylitis is a defined clinical syndrome with infection of the free thoracic and adjacent vertebrae, resulting in chronic osteomyelitis and spinal cord compression. Clinical signs of leg paresis often develop in broilers >35 days old.

Osteomyelitis and arthritis are detected on gross examination by examination of articular surfaces and bone physes. Detection of subtle lesions requires histopathology. Affected joints are often swollen with fibrinous or caseous exudate. Bones have areas of lysis and/or replacement by caseous exudate, most often within the physeal region.

Response to treatment with antibiotics currently approved for use in poultry is often poor for bacterial bone and joint infections. Antibiotics may be used to control the bacteremia contributing to new cases and to modify the bacterial flora within a flock. When individual birds are of high value, long-term antibiotic therapy may improve some less severe cases. Control requires minimizing sources of infection and stock susceptibility.

Viral Arthritis

Osteopetrosis

Osteopetrosis in chickens is due to infection with specific strains of avian leukosis virus (see Lymphoid Leukosis in Poultry). Growth and differentiation of osteoblasts is altered by the virus, resulting in diaphyseal and/or metaphyseal, periosteal, and circumferential accumulation of woven and lamellar bone. Osteopetrosis is bilaterally symmetrical and involves the long bones, especially the tibiotarsus and tarsometatarsus. Birds 8–12 wk old are most commonly affected. Lymphoid leukosis often occurs in chickens with osteopetrosis. Avian osteopetrosis differs from mammalian osteopetrosis in which a defect in osteoclast function results in abnormal bone resorption and accumulation of primary spongiosa in the marrow cavity.

Amyloid Arthropathy

Extensive amyloid arthropathy is primarily caused by Enterococcus faecalis or Mycoplasma synoviae. Clinical cases are seen only occasionally and most frequently affect the hock joint. Brown layer chickens are particularly susceptible. Some cases have been attributed to contamination of a previously sterile vaccine diluent with E faecalis during administration (eg, Marek's vaccine in day-old chicks). Joints may be enlarged by yellow-orange material in the articular space.